The prior art includes filters using the principle of electrostatics for removing particles from various gasses, normally air, at velocities up to 10 m/s. The principle here employed is as follows. The air is propelled through an electric field where particles in the air receive an electric charge. The charged particles move into a collector section where each alternate plate is charged with the same polarity as the particles, and repels them. The other set of plates are grounded, which collect the particles. The remaining air, cleaned of the majority of particles, is then re-introduced into the environment. The contaminated plates are cleaned by washing, normally by water/detergent, high pressure air or other means. The particles can be charged positively or negatively depending on the environment and the location of the filter. While the electrostatic filter has evolved over the years there remains two basic operational problems. As the air speed increases so the efficiency decreases and two as the air speed increases so the pressure drop increase so that the running cost becomes prohibitive. It has been acknowledged that at speeds over 10 m/s the filter is no longer feasible to operate.
German publication DE 1457325 discloses an apparatus that utilizes both electrostatic and centrifugal forces. An electrode is arranged centrally in a tube. Gas is driven into helical motion by a tangential inlet arrangement to the tube and by the provision of helical tracks around the centre electrode.
A similar device is disclosed in DE 717477, comprising a central electrode and blades in a tube, for utilizing both electrostatic and centrifugal forces.
WO 99/61160 discloses an arrangement in which a propeller blade at the inlet provides a spiral air flow inside a chamber with a central electrode.
A common feature of all these prior art solutions with center electrode, is that a special air spin generator (tangential gas inlets, special spiral track devices or propellers) is necessary. Further, in devices with a central ionising electrode, this electrode is then a wire situated axially along the tube. The main problem with a wire is that it breaks. This is caused by wire vibrations due to high voltages. The wire vibrates even when there is no air movement.
Hence, there is a need of a strong and self-stabilized center electrode, and generally there is a need of a simpler and less expensive construction than disclosed by the prior art.